The main reason why IE 8 beat out the competition -- which included Apple Safari 4, Google Chrome 4, Mozilla Firefox 3.5 and Opera 10 -- appears to be Microsoft's use of its "SmartScreen Filter" technology. SmartScreen is a reputation-based URL comparison service that warns users of known threats, such as a Web page that attempts to get users to download malicious programs. Chrome, Firefox and Safari all used Google's "Safe Browser feed" service instead. The report did not explain what URL reputation service was used by Opera.

According to the report, IE 8 caught 85 percent of live threats. Other browser fell way behind in protection against socially engineered malware. Safari caught 29 percent of live threats, tying with Firefox. Chrome caught 17 percent, while Opera caught less than 1 percent.

Opera finished dead last in this report's overall comparisons of protection against socially engineered malware. This report is actually NSS Labs' third release on the subject, and Opera similarly trailed in the previous reports, published on July 20, 2009 (PDF) and March 12, 2009 (PDF). Back in March 2009, an Opera Software blog described NSS Labs' report as "just another Microsoft marketing trick." The blog questioned NSS Lab's methodology and suggested that statistical tricks were used.

NSS Labs methodology for the February study is described as a "proprietary Live Testing" approach. The objective is to insert the freshest samples of malware into the testing process over a set period of time. It's an approach that software security vendor Trend Micro announced support for late last year.

The report measured browser protection against malware only when spread by social engineering techniques. It excluded other means of spreading viruses, trojans and worms. Consequently, just 562 URLs passed the NSS Labs' criteria and were used in the study.

Browsers that scored well on the tests essentially had to show protection against trickery used by hackers to get users to click on a link or visit a malicious Web page, thereby downloading a malicious program. Malware associated with browser plug-ins (also called "add-ons") was excluded from the report. The report also did not test for "clickjacking or drive-by downloads."